More on comfort noise fun and games and T1 circuits:
Digital phone systems by their nature are dead silent unless audio is being received. Sample audio at something above Mr. Nyquist's calculated rate to reconstruct it successfully and pass that to the other side. No audio, send no bits.* To people who had used the old PSTN POTS analog service which had some noise on the line, it sounds "weird" or "too quiet when no one is talking".
People thought their phone call had gotten disconnected since dead silence was a sign that the call had dropped in the network in the past. People would constantly say "hello, hello?". So early VoIP engineers added "comfort noise". White noise generated at either end at a low level to make the listener know the call was still up.
Cell phones often are silent when no one is speaking and you see people doing the same thing. Some handsets generate comfort noise also. Early Cisco gear turned comfort noise on by default.
My experience with it was VoIP gateways at carriers that had it turned on in BOTH directions which would REALLY **** off the voting algorithms in the DSPs of phone conferencing systems. Especially if the conferencing system had Automatic Gain Control (AGC) enabled. The AGC would amplify the comfort noise up thinking it was someone talking with low audio and then the voting algorhythm would vote the channel/line with comfort noise into the second "talk slot". The result is you hear the speaker who's louder than the comfort noise and every time they stop talking there's a whooshing white noise sound until they start again. If the person on the line with comfort noise speaks, the whooshing goes away. Or usually more likely, multiple lines do it simultaneously and the voter algorhythm is constantly switching between lines for the second "talk slot" and popping two lines that are hissing away in together when no one is talking. It sounds like crap.
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*In reality you never send all zeros or all ones across a digital circuit at the low-level because in the old days you needed the voltage swings to keep the circuit timing going. There is bit-flipping going on in a T1 or any other PSTN style synchronized clock circuit at the physical level, but the devices do it and then put it back internally to the real data stream being sent. These are defined in the SF/AMI and ESF/B8ZS circuit algorhythms.
SuperFrame/Alternate-Mark Inversion, where one and zeros were called "Mark" and "Space", so inverting every other "Mark" was done to keep the voltage swing going if all ones or all zeros was sent.
Extended SuperFrame/Bit-8 Zero Suppression. Same idea after the 8-bit version of the T1 circuit was developed.
SuperFrame/AMI robbed bits to indicate alarms. Stealing the second-most-significant bit and flipping it in a pattern to tell the far-end that something is wrong with the signal received ("Yellow Alarm"). Red Alarm is a complete loss of signal, Blue Alarm is all ones being received. This allowed for troubleshooting.
ESF utilized the ability of better circuits to stay synchronized with less need for sync bits (PLL circuits originally) to move the Alarm bits out of the analog channel bits, stop bit-robbing those and rob-bits from the framing/clocking bits.
Feed an analog conference bridge fed with T1 circuits that are SuperFrame/AMI, put all 24 lines into a conference call and then whistle into a phone. The second most significant bit will be set by the high pitched tone for enough frames that the far end will think the T1 is in Yellow Alarm. A telco switch receiving Yellow Alarm is going to drop all calls on that circuit. (And long ago would have marked the circuit as down and delivered no more calls to it without manual intervention, but nowadays they reset as soon as the Alarm indication has cleared.
You just flushed your conference call. Bye!
SF/AMI is hard to order these days. When I started in conferencing some carriers had old gear that couldn't deliver ESF/B8ZS circuits and we had to budget to place the 24th phone line in a permanently off-hook/disabled state so the above "flush" wouldn't happen.
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